1. Field of the Invention
The present invention relates to a measuring instrument. More specifically, the present invention relates to a measuring instrument for measuring dimensions of a workpiece by advancing/retreating a spindle thereof, the measuring instrument being typified, for instance, by a micrometer and a micrometer head.
2. Description of Related Art
Conventionally, a measuring instrument, such as a micrometer or a micrometer head, used for measuring dimensions of a workpiece by advancing/retreating a spindle in accordance with its screwing rotation has been known (refer to, for instance, Document 1: Japanese Utility Model Laid-Open No. S49-80260, and Document 2: Japanese Patent Laid-Open Publication No. S54-130152).
Such a measuring instrument includes a main body having a female thread, a spindle having a lead screw engaged with the female thread of the main body, a rotation detector which detects a rotation of the spindle, and an arithmetic processor which obtains a displacement of the spindle based on a signal output from the rotation detector.
In such a measuring instrument, the displacement per one rotation of the spindle is defined by a screw pitch of the lead screw formed on the spindle, the screw pitch of the lead screw generally being 0.5 mm, for instance.
The rotation detector includes, for instance, a rotor which rotates together with the spindle, a stator fixed on the main body in a state of being opposed to the rotor, and a phase calculator which increments a signal output from the stator corresponding to the rotation of the rotor to calculate a rotational phase of the rotor.
FIG. 11A and FIG. 11B show confronted surfaces of a rotor 1 and a stator 2.
As shown in FIG. 11A, the stator 2 includes a transmitting terminal 21 which has a plurality of electrode plates arranged at equal angular interval on a predetermined circumference on a surface opposing to the rotor 1, and a ring-shaped receiving terminal 22 arranged concentrically to the transmitting terminal 21. An AC signal whose phase is modulated is transmitted from a pulse modulator to each electrode plate of the transmitting terminal 21. The transmitting terminal 21 includes sixteen electrode plates, to which AC signals are respectively applied, the phases of the AC signals being different by 45 degrees with each other. As shown in FIG. 11B, the rotor 1 includes a coupled terminal 11 which straddles the transmitting terminal 21 and the receiving terminal 22 of the stator 2 on a surface opposing to the stator 2. The coupled terminal 11 is electrostatically coupled with a predetermined number of electrode plates among the electrode plates of the transmitting terminal 21.
With such a configuration, when the spindle is rotated, the spindle is advanced and retreated along with screwing rotation of the spindle against the main body. The rotation of the spindle at this time is detected by the rotation detector. In other words, when the spindle rotates, the rotor 1 rotates together with the spindle.
Further, when the predetermined AC signal is transmitted to respective electrode plates of the transmitting terminal 21, the potential is transmited in an order of: transmitting terminal 21 to coupled terminal 11 to receiving terminal 22. Since the electrode plates of transmitting terminal 21 electrostatically coupled with the coupled electrode 11 change when the rotor 1 is being rotated, the potential of the receiving terminal 22 electrostatically coupled with coupled terminal 11 will change too. A rotational phase of the rotor can be obtained by sampling the potential of the receiving terminal 22 at a predetermined sampling pitch to obtain pulse signals and count the obtained pulse signals by a phase calculator.
Since the rotational phase of the rotor 1 equals to a rotational phase of the spindle, the displacement of the spindle can be calculated by the arithmetic processor based on the rotational phase of the spindle and the pitch (0.5 mm, for instance) of the lead screw.
However, in the case where the screw pitch of the male screw formed on the spindle is 0.5 mm or 0.635 mm, since the displacement per one rotation of the spindle is small, the spindle has to be rotated for many times every time an object to be measured is changed, which raises a problem in operational performance.
A method to increase the displacement of the spindle per one rotation is, for instance, to increase the pitch of the lead screw up to 1 mm to 2 mm so as to increase an advancing/retreating amount per one rotation of the spindle.
By increasing the pitch of the lead screw up to 1 mm to 2 mm, since the advancing/retreating amount of the spindle per one rotation is increased, there will be no doubt that the operational performance can be improved due to increased displacing speed of the spindle.
However, when the pitch of the lead screw is increased, detection accuracy and detection resolution of the rotation detector have to be improved correspondently. For instance, when the pitch of the lead screw is increased by 4 times, if the detection resolution for the phase remains unchanged, the detection resolution for the displacement of the spindle will simply be reduced to ¼.
In order to increase the detection resolution per one rotation of the spindle, a feasible method is to reduce sizes of the electrode plates of the transmitting terminal 21 to increase the number of the electrode plates, so that minute change of the rotation angle of the rotor 1 can be detected. However, when the number of the electrode plates of the transmitting terminal 21 is increased, since the electrode plates of the transmitting terminal 21 electrostatically coupled with the coupled terminal 11 vary when the rotor 1 is rotated by minute angle, variation period of the potential of the receiving terminal 22 becomes short.
Thus detection errors such as skip-read of the pulse signal will frequently occur in the phase calculator, which will cause a problem that angle variation of the rotor 1 can not be accurately caught up.
A feasible solution to the problem is to shorten a sampling period at which the potential of the receiving terminal 22 is sampled, however the room for shortening the sampling period is limited depending on a speed of an IC and the like. Further, if the sampling period is shortened, since the pulse signal becomes easily affected by disturbance such as noise, detection accuracy will unexpectedly decrease due to electric disturbance from outside. Further, if the sampling period is shortened, power consumption will increase, so that the battery service life will be reduced.
Since it could not be achieved to detect the displacement of the spindle at high accuracy and high resolution with a spindle having a high lead screw, it is desirable to provide a measuring instrument whose spindle can be displaced at high speed, and therefore has good operational performance as well as high detection accuracy and high detection resolution.